Topanga Lawrence from "Boy Meets World" was around 12 years old at the start of the show, which roughly corresponds to the beginning of the junior high school period in the series. Over the course of the show, her age progresses, and in Season 4 she celebrates her 16th birthday. The series timeline is set mostly between ages 12 to 17 or 18 as the characters grow through middle and high school grades.
To clarify, the actress Danielle Fishel who portrayed Topanga was born on May 5, 1981, and began the role when she was about 12 years old herself, aligning with the character's age at the show's start.
So, Topanga’s age in the show starts at about 12 years old and goes up to around 16 or 17 years old during the main run of "Boy Meets World."
